PA is the altitude in standard atmosphere at the pressure measured at site - it gives you an idea of how how an aircraft will perform aerodynamically, so necessary for take-off and landing performance calculations. Also an indication of how a jet engine performance will be affected, as jet engines are just lots of whirling wings..
DA is the altitude in standard atmosphere at the density measured at site - primarily is a measure of how the pilot will feel in a non-pressurised aircraft as it correlates closely to oxygen partial pressure, and is also relevant for performance of non-turbo piston engines.
OK, its not an exact science, but more or less without going into lots of detail that is how the figures can be interpreted..